Self-propelled work machines, such as motor graders, are often required to operate on uneven terrain or in other poor traction conditions. To provide better traction, these machines are often equipped with an all-wheel drive (AWD) system. In poor traction conditions, the front wheels of an AWD machine, normally used for steering, may also be driven to increase traction. For example, a motor grader may be operated in an AWD mode in order to obtain maximum traction when grading on a side slope or removing snow from a roadway.
However, in certain traction conditions, such as in snow or on loose earth, AWD machines may experience a bouncing condition known as “hop.” Hop may occur when the driven front wheels of the machine experience poor traction conditions that cause them to alternately spin, thus making a depression in the surface, and then stick, thus thrusting the front of the machine upward as the front wheels climb the forward wall of the depression that was just made. If the force of the thrust is great enough, the front wheels of the machine may actually hop off the ground, though this is not necessarily the case. When the weight of the front of the machine comes back down, the front wheels may create another depression, thus causing the machine to hop again. These hops may become resonant at a particular low frequency (e.g., 2-3 Hz), depending upon the configuration of the machine.
Hop has several undesirable effects. For example, hop may cause the machine to lose traction, and thus operate inefficiently. Also, it may result in excess force being applied to the front wheel suspension as the machine hops. Further, excess hop may cause damage to the surface that the machine is preparing, e.g., by causing a ground-contacting implement of the machine (such as a grader blade) to bounce as it travels across the surface.
Prior art systems for mitigating hop have been developed. One such system is described in U.S. Pat. No. 5,474,147 to Yesel et al. In this system, the front wheels of the machine are driven by hydraulic motors. Machine hop is detected by sensing fluctuations in hydraulic motor pressure. If fluctuations greater than a certain magnitude and frequency are detected, then a controller decreases the hydraulic motor torque by a predetermined amount.
However, the system described in the '147 patent may be triggered by non-resonant hop, thus leading to inefficient operation of the machine. Further, the controller in this system does not decrease the torque in proportion to the magnitude of machine hop. Consequently, the system may be required to step-down the hydraulic motor torque several times in succession in order to eliminate hop. This may cause the machine to lose speed suddenly, which may be disconcerting to the operator of the machine.
The presently disclosed hop mitigation system is directed to solving one or more of these shortcomings of the prior art hop mitigation systems.